Fig. 1. Scattering effect between surface wave and defect

Fig. 2. Experimental diagram

Fig. 3. Laser ultrasonic platform

Fig. 4. Experimental samples

Fig. 5. Sample size

Fig. 6. Direct wave spectrum diagram

Fig. 7. B-scanning images of surface defect. (a) Defect depth of 0.40 mm; (b) defect depth of 0.30 mm; (c) defect depth of 0.20 mm; (d) defect depth of 0.10 mm; (e) defect depth of 0.08 mm

Fig. 8. Frequency spectrum energy diagrams of reflection/transmission signals. (a) Defect depth of 0.40 mm; (b) defect depth of 0.30 mm; (c) defect depth of 0.20 mm; (d) defect depth of 0.10 mm; (e) defect depth of 0.08 mm (red solid lines and blue solid lines represent curves of transmission surface wave and defect reflection surface wave, respectively)

Fig. 9. Relationship between surface defect depth and corresponding wavelength

Fig. 10. B-scanning images at different distances between excitation point and detection point

Fig. 11. Frequency spectrum energy diagrams of reflection/transmission signal when distance between excitation point and detection point is different. (a) Distance of 5 mm; (b) distance of 10 mm ; (c) distance of 15 mm; (d) distance of 20 mm; (e) distance is of 25 mm (the red solid lines and blue solid lines represent curves of transmission surface wave and defect reflection surface wave, respectively)

Fig. 12. Samples with different materials. (a) Steel sample; (b) aluminum sample

Fig. 13. Frequency spectrum energy diagrams of reflection/transmission signals of samples with different materials. (a) Aluminum; (b) steel (the red solid lines and blue solid lines represent curves of transmission surface wave and defect reflection surface wave, respectively)

Table 1. Defect depth

Table 2. Measured location and error of defects

Table 3. Measured defect depth and error

Table 4. Measured defect depth and error at different distances between excitation point and detection point

Table 5. Defect depth and error of samples with different materials